Traditionally the preparation of very fine grained, dense alpha alumina has been difficult. With the exception of diaspore (beta alumina monohydrate), the firing of other alumina precursors such as basic aluminum salts, aluminum hydroxides or boehmite (alpha alumina monohydrate) to generate alpha alumina involves first the formation of intermediate, metastable transition alumina phases. The transition aluminas, although possessing very fine microstructures, convert to alpha alumina with considerable coarsening, and often require very high temperatures for densification.
U.S. Pat. No. 3,808,015 discloses the preparation of microcrystalline alpha alumina ceramic fibers by the extrusion of a concentrated slurry of fine particles of alpha alumina in an aqueous phase which contains a precursor of alumina followed by firing to at least 1400.degree. C. to convert the fibers to dense alpha aluminum oxide. The concentrated slurry of alpha alumina particles was taught as necessary to overcome the problems of fiber shrinkage during firing and the problem of filament-to-filament sticking during the spinning and the firing steps. These fibers, however, are limited in strength and flexibility by the presence of relatively large flaws which could have resulted from the imperfect dispersion of the alumina particles or from the presence of large particulate alumina in the spin mix. These fibers are also characterized by a microscopic roughness height between about 1,100 and 7,000 Angstroms. This rough surface makes handling procedures such as weaving difficult and reduces the flexibility of woven articles derived therefrom. More recently, Kumagai and Messing [Comm. Am. Ceram. Soc. C-230 (1984) and J. Am. Ceram. Soc. 68(9), 500 (1985)] reported that the seeding of boehmite sols with fine alpha alumina powder resulted in enhanced densification at modest firing temperatures (1200.degree. C.), and enabled the preparation of dense, submicron microstructures. Both of these processes utilized the addition of particulate crystalline alpha alumina to enable the production of dense, microcrystalline alpha alumina. It is believed that the finely divided alpha alumina provides templates for the epitaxial growth of alpha alumina from the transition alumina.
Suwa et al., Journal of Materials Science Letters 5, 21-24 (1986) also disclose the use of particulate alpha Fe.sub.2 O.sub.3 as having a minor effect in seeding the alpha alumina transformation in aluminas prepared from boehmite sols.
G. C. Bye and G. T. Simpkin, J. Amer. Ceram. Soc., 57(8), 367 (1974); Y. Wakao and T. Hibino, Nagoya Kogyo Gijutsu Shikensho Hokoku, 11, 588 (1962) have shown that the doping of alumina with small amounts of metal ions such as iron(III), and chromium(III) can lower the temperature at which the transition aluminas are converted to the alpha phase. These ions have ionic radii and charge densities similar to that of Al.sup.3+ and also form sesquioxides isomorphous with alpha alumina. The hydrous oxides of these metals convert to the alpha form at much lower temperatures than the corresponding hydrous aluminas. The doping of alumina with these metal ions, however, has not resulted in the generation of dense, microcrystalline alpha alumina.
Because of the high modulus, strength, and chemical and high temperature resistance of alpha alumina, fibers of this material are desired for application as high temperature filtration media, refractory insulation, and structural composite reinforcement. A very fine microstructure is requisite for the production of strong, non-friable ceramic fibers. In applications where a textile quality fiber is desired, such as in the production of articles which require weaving, a smooth fiber surface is desirable. In addition to application in ceramic fibers, fine structured alpha alumina of high density has been found to perform as a superior abrasive.